In the manufacture of integrated circuits, photolithography techniques are used to form integrated circuit patterns on a substrate. Typically, the substrate is coated with a photoresist, portions of which are exposed to ultraviolet (UV) radiation through a mask to image a desired circuit pattern on the photoresist. The portions of the photoresist left unexposed to the UV radiation are removed by a processing solution, leaving only the exposed portions on the substrate. These remaining exposed portions are baked during a photostabilization process to enable the photoresist to withstand subsequent processing.
After such processing, in which the integrated circuit components are formed, it is generally necessary to remove the baked photoresist from the wafer. In addition, residue that has been introduced on the substrate surface through processes such as etching must be removed. Typically, the photoresist is "ashed" or "burned" and the ashed or burned photoresist, along with the residue, is "stripped" or "cleaned" from the surface of the substrate.
One manner of removing photoresist and residues is by directing a microwave-energized plasma at the substrate surface. Typically, the plasma is formed by a gas mixture that is transported through a plasma tube that passes through a microwave cavity. Microwave energy within the cavity is introduced into the plasma tube to excite the gas mixture therein and form a plasma. The plasma passes from the tube into a process chamber in which resides a photoresist-coated semiconductor substrate to be ashed.
One type of gas mixture that is used to generate the plasma is oxygen-based. For such a gas mixture, a quartz plasma tube is suitable and provides an efficient means for transporting the plasma to the process chamber. Other types of materials used for constructing the plasma tube are less efficient in transporting the plasma to the process chamber. For example, if a sapphire tube is used, atomic oxygen in the plasma recombines with the inner surface of the sapphire tube, reducing the amount of atomic oxygen available for the ashing process.
For certain ashing and other plasma-related processes (e.g., residue removal), it has been found that a source of fluorine may be added to the process gas mixture constituency to provide for more effective or efficient processing (e.g., enhanced ash rates). Such improved ash rates (and residue removal capabilities of fluorine), however, are achieved at the expense of degradation of the quartz plasma tube. The degradation is caused by the fluorine in the process gas mixture that etches the inner surface of the quartz tube. The use of a sapphire tube for a fluorine-oxygen based plasma will prevent fluorine etching of the inner surface of the tube. However, the use of a sapphire tube for such a plasma reintroduces the problem of atomic oxygen recombination with the inner surface of the sapphire tube. In addition, it has been found that ash rates of photoresist using a sapphire plasma tube when using non-fluorine chemistries are markedly lower than ash rates observed when using similar gas flows in a quartz plasma tube.
Accordingly, it is an object of the present invention to provide a plasma source for a semiconductor substrate processing system, such as a plasma asher, that permits the use of oxygen-fluorine plasma chemistries that suffers from neither (i) fluorine degradation of the plasma-carrying transport tube or (ii) atomic oxygen recombination with the surface of the sapphire tube, while providing suitable photoresist ashing rates. It is a further object of the present invention to provide a switching mechanism such that using the same asher, a user may select between (i) non-fluorine chemistries, using a quartz-like plasma tube in which the plasma is generated to avoid the deleterious effects of oxygen recombination, thereby achieving suitably high ash rates, and/or (ii) fluorinated chemistries, using a sapphire-like tube in which the plasma is generated tube to avoid the deleterious effects of etching.